Among the three predominant international standards for the 3-rd generation mobile communication system, the TD-SCDMA is the only one which adopts the Time Division Duplex (TDD). The TD-SCDMA supports asymmetric service transmission in the uplink and downlink and has more flexibility with respect to the utilization of the frequency spectrum.
An existing TD-SCDMA system is a system provided with a chip rate of 1.28 Mcps and a bandwidth of 1.6 MHz. The same signal format is used for a data part and uplink and downlink pilot parts of the system. FIG. 1 illustrates a structural form of a frame in the existing TD-SCDMA system. In FIG. 1, each sub-frame is further divided into seven normal timeslots and three special timeslots. The three special timeslots are DwPTS, GP and UpPTS, respectively. Among the seven normal timeslots, TS0 is usually allocated to the downlink while TS1 is usually allocated to the uplink. An uplink timeslot and a downlink timeslot are separated with a transition point. The DwPTS timeslot is intended for transmission of a downlink pilot for an initial search in a cell, the UpPTS is intended for transmission of an uplink access timeslot for a random access signal, and the GP acts as a guard interval for transition of the downlink timeslot to the uplink timeslot to prevent mutual interference between uplink and downlink channels and has a length which determines the maximum of a coverage radius of the cell.
In the prior art, terminals enable a random access by sending a random access sequence in the UpPTS timeslot, and the random access sequence may vary from one terminal to another. However, the terminals of all users have to occupy the same frequency band during transmission of the uplink random access sequence due to the bandwidth limitation of the TD-SCDMA system.
With the development of technology, people who pose increasingly higher demands on mobile communication wish the system to provide data transmission services with a large capacity, a high rate and a low time delay. In order to cater to the increasingly growing demands, the TD-SCDMA system has to evolve and improve the performance continuously. A technical standard for the TD-SCDMA evolvement system is currently being established in the Long Term Evolvement (LTE) research project under the 3GPP organization, and this technical standard is also becoming a predominant technology for the LTE TDD system. A system enabling a service with a high rate, a large capacity and a broader occupied bandwidth has been proposed in the evolvement solution of the TD-SCDMA and is referred to as a wideband time division duplex cellular system. In the wideband time division duplex cellular system, the bandwidth can be up to 200 MHz or more, and a terminal can be supported to operate over a frequency band of 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz and 20 MHz.
Random access is an important function of a cellular mobile communication system. The purpose of the random access is to enable an efficient access of a terminal device in a cellular network to the cellular network, accomplishing allocation of a transmission channel and communication transmission of data. In the TD-SCDMA and its evolvement system, a terminal can accomplish synchronization with the system and power control by a random access. Further for the TD-SCDMA evolvement system, with the random access, the terminal can adjust the time to transmit data in accordance with a distance from a base station so as to control the synchronization precision within a cyclic prefix (CP) length, thereby reducing mutual interference between the terminal and other terminals. Meanwhile, the random access enables the base station to control the power of the terminal to transmit a signal in accordance with the distance from the terminal so as to reduce interference between cells. The random access is a principal step for the terminal to communicate, and the terminal gains an access to the cellular network system through this process to obtain an uplink channel resource for transmission.
In the wideband time division duplex cellular system, if the random access is implemented by the code division multiple access in the existing TD-SCDMA system, the increased number of access users may result in a problem that the probability of collision of the random sequences increases since the users may select the same random sequence. Furthermore, serious interference between terminals may arise because all the users occupy the same channel resource. Consequently, how to determine a random access mechanism and design a random access channel is a pressing technical problem to be solved.